The present invention is in a process for the direct reduction of iron oxide-containing materials in a plurality of series-connected fluidized beds by means of solid carbonaceous reducing agents, wherein weakly reducing conditions are maintained in the first fluidized bed and strongly reducing conditions are maintained in the second fluidized bed.
In the mining of ores and in the processing or relatively poor iron ores, fine-grained ores become available in increasing quantities. Then fine-grained ores can be directly reduced below their melting temperature by particularly fluidized bed processes because these processes provide for highly favorable conditions for a transfer of matter and heat. In the direct reduction by means of solid carbonaceous reducing agents, the following different steps must be performed in succession: heating the ore and coal, devolatilizing the coal, gasifying the coal, reducing the ore and possibly cooling the end product.
German Patent Publication No. 22 53 228 discloses a process in which fine coal is used as a reducing agent for a direct reduction resulting in a metallization of 60 to 80%. The reduction is effected in two connected orthodox fluidized beds in series. The fluidized beds are only slightly enlarged or expanded and have a defined bed surface. Ore and coal are charged into the first fluidized bed and are fluidized with preheated air so that a gas having a small reduction potential is produced. This results in a reduction of the iron oxide content mainly to the FeO stage. The weakly reducing exhaust gas from the first fluidized bed is supplied to a steam generator and the pre-reduced material is introduced into a second fluidized bed. The second fluidized bed is also supplied with preheated air as a fluidizing gas. A strong reducing gas is produced in the second fluidized bed so that the desired metallization is effected.
The reduced product flows into a smelting reactor, which is supplied with the required heat substantially in the form of electrical energy. The strong reducing exhaust gas is also supplied to the steam generator. In that process, hot spots formed in the second fluidized bed result in temporary molten phases, which tend to form crusts on the air inlet nozzles so that the uniform progress of the operation is severely disturbed.
Published German Application No. 25 10 116 discloses a process in which a direct reduction is effected in a circulating fluidized bed by means of solid carbonaceous material. The fluidized bed reactor comprises two stages, which are arranged one over the other and are not separated from each other. Ore and coal are charged into the intermediate portion of the reactor and oxygen-containing gas in also injected into said intermediate part. The required heat is generated by a partial combustion of the carbonaceous material in the upper portion of the reactor so that the carbonaceous material is coked and devolatilized and iron oxide is reduced to a certain degree at 850.degree. to 1000.degree. C. A suspension of solids and gas is withdrawn from the upper portion. The solids are separated from the suspension and recycled to the intermediate portion of the reactor. After CO.sub.2 and water vapor have been removed from the gas, the latter is supplied as a fluidizing and reducing gas into the lower portion of the reactor. Reduced material and coke are withdrawn from said lower portion. The problems mentioned above arise also in the last-mentioned process.
It is an object of the invention to ensure in the direct reduction by means of solid carbonaceous reducing agents in a fluidized bed that a formation of hot spots and crusts, particularly at the nozzles used to supply oxygen-containing gases, will be avoided and a satisfactory sequence of operations will be effected.